Adding DataLoader2 class as future replacement of DataLoader (#63742)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63742
Supports sharding and batching on loader level**
Supports sharding and batching on loader level
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D30494506
Pulled By: VitalyFedyunin
fbshipit-source-id: 6648e09d955055ac38e3a4e3973f701acefca762
